1. Field of the Invention
The present invention relates to a parameter setting method and a parameter setting device of a model which approximates circuit element characteristics when a circuit operation is tested. In addition, the invention relates to a circuit operation testing method and a circuit operation testing device.
2. Description of the Related Art
A circuit operation test (also referred to as a circuit simulation) is performed for the purpose of finding a defect in a circuit design as soon as possible and predicting performance. A designed circuit is modified or improved based on a calculation result of a circuit operation test. When a circuit operation is tested, a parameter obtained by a model for indicating characteristics of an element included in a circuit is required. A parameter obtained by a model is determined depending on a manufacturing process or a size of a device, or determined by using software (hereinafter referred to as a tool) for extracting a parameter in accordance with electric characteristics obtained by measuring a device.
For a circuit operation testing program, for example, SmartSpice® (manufactured by Silvaco International), or the like can be used. For software (tool) for extracting a parameter, UTMOST (manufactured by Silvaco International) can be used. Besides, products marketed by Synopsys, Inc. and Cadence Design Systems, Inc. are given.
For a circuit operation test of a thin film transistor (hereinafter referred to as TFT), a model according to a physical phenomenon indicating electrical characteristics that might affect a TFT, which should be considered, is used. As such a model, there is a classical model called gradual channel approximation or depletion layer approximation, an RPI model, or the like. In the aforementioned SmartSpice®, a LEVEL number is given for each model. A model of a large LEVEL number tends to have high physical precision and a large number of parameters. Meanwhile, a model of a small LEVEL number has a small number of parameters, but physical precision thereof is low. For example, improved depletion layer approximation is LEVEL 2, and an RPI model is LEVEL 36.
In order to extract a parameter obtained by a model used for a circuit operation test, the model is required to be determined by obtaining measurement data. Selection of a model is significant for a circuit operation test. A parameter is extracted based on a nonlinear least squares method in a process of software. In the nonlinear least squares method, various factors are necessarily considered, such as handling in the case of having a plurality of minimum values, a determination condition of a loop concerning convergence, and a processing method of values, and a lot of devices are required for them. The various factors are unique properties of software (tool) for extracting a parameter; therefore, even when the same measurement data is used, a different value is extracted depending on software (tool). In the case of a simple model, a model formula can be expressed by a linear coupling of a parameter, so that a calculation can be performed without using marketed software (tool). The calculation result is led having some differences in calculation quantity in the calculation process, which are notlarge, by an algorithm.
In the nonlinear least squares method, in principle, an enormous quantity of calculations is required to be performed for obtaining a proper value. When a parameter is actually extracted by using a model of LEVEL 36, it takes approximately 10 minutes for data of one curve. However, when a parameter is extracted by using a model of LEVEL 2 for example, not so many linear portions are included. Therefore, simple software (tool) can extract the parameter, and even data of 100 curves can be extracted in just a few seconds.